1. Field of the Invention
This invention relates to a powerline carrier control system having a powerline carrier control transmitter which generates a keyed AC output voltage at audio frequency. The output voltage is fed, via a coupling unit, into an AC power supply network having an established line frequency.
2. Discussion of the Prior Art
By using a powerline carrier control system, information can be transmitted to a consumer by using a single or multi-phase electric power supply network as the transmission path. Such information may relate, for instance, to initiation of switching operations in the supply network, switching consumers' meters to different rates (e.g., a night rate), or informing a certain group of people (e.g., the fire department).
Such a powerline carrier control system consists, essentially, of a powerline carrier control transmitter, a coupling unit for feeding into the network, and one or more powerline carrier control receivers which are connected to the supply network and control or otherwise inform the consumers. The powerline carrier control transmitter generates an audio-frequency AC output voltage which is keyed in accordance with the information to be transmitted. It is superimposed by means of the coupling unit on the line voltage in the supply network. The powerline carrier control receivers connected to the supply network are selective and decode the transmitted signals and control the consumer connected thereto. The line frequency is usually 50 or 60 Hz, and a frequency of between 150 and 750 Hz is usually used as the audio frequency. The audio frequency is therefore distinctly higher than the line frequency.
A powerline carrier control system of this type is described, for instance, in U.S. Pat. No. 4,021,797. The powerline carrier control system described there uses, as the powerline carrier control transmitter, a static converter, consisting of a line-fed diode rectifier, an intermediate voltage link, and a three-phase pulsed inverter connected thereto which is constructed using power thyristors. The pulsed AC output voltage of the pulsed inverter, the fundamental frequency of which is in the audio range, is fed into the three-phase AC supply network via a coupling unit. The coupling unit used here consists specifically of a series coupling unit, and more specifically, of a combination, designated as a "resonant shunt", of coils and capacitors as well as three coupling transformers connected thereto, which are delta-connected on the primary side and are designed like current transformers. The "resonant shunt" contains three delta-connected capacitors, the junction points of which are each connected, via a choke, to the three output leads of the pulsed inverter. This L-C combination is series resonant at the line frequency; it prevents line-frequency voltages from being coupled back into the powerline carrier control transmitter from the network side. The " resonant shunt" further contains three other delta-connected capacitors, the junction points of which are connected directly to the three output leads of the pulsed inverter. Together with the mentioned L-C combination of the series resonnant circuit, these capacitors form a parallel resonant circuit at the audio frequency.
Another powerline carrier control system of the type mentioned at the outset is described in the journal "Elektizitatswirtschaft", Vol. 70 (1971), No. 9, pages 237 to 241, particularly FIG. 1. There, a converter consisting of a line-fed diode rectifier, intermediate DC voltage circuit, and a line-commutated inverter using thyristors in a three-phase bridge circuit serves as the powerline carrier control transmitter. Here a series coupling unit is also used as the coupling unit, it being stated that the coupling unit can also be designed as a parallel coupling unit. The series coupling unit comprises, for each phase, a first L-C series circuit, a second L-C series circuit and a coupling transformer. In order to keep line-frequency back voltage away from the converter, the AC output voltage of the inverter is fed to the coupling transformer via a series resonant circuit tuned to audio frequency. A second series resonant circuit forms a short circuit for the line frequency directly at the output of the inverter. The main inductance of the coupling transformer is in parallel resonance with the mentioned capacitor for the audio frequency.
A powerline carrier control system using a parallel coupling unit is described in VDI-Zeitschrift, Vol. 101 (1959), pages 733 to 768, especially from FIG. 17, and from "Siemens-Zeitschrift" 48 (1974), pages 69 to 75, particularly FIGS. 2 and 3.
In the known coupling units, special attention must be given to the design and the tuning of the individual components so that no line-frequency back voltage appears on the transmitter side. A line-frequency back voltage would cause equalization or short-circuit currents and would thereby endanger the components of the powerline carrier control transmitter. The chokes and capacitors used as well as the coupling transformers must be of high quality. This applies to powerline carrier control systems using series as well as parallel coupling units. The cost for the coupling unit has, up to now, been considerable. In some of the powerline carrier control systems built so far, the cost of the L-C combination is approximately equal to the cost of the coupling transformers and also approximately equal to the cost of the powerline carrier control transmitter itself.
In German Offenlegungsschrift No. 23 04 734, a powerline carrier control system having a thyristor powerline carrier control transmitter is described which is connected to the power network via transformers alone. One resonant shunt is eliminated entirely by controlling the thyristor inverter so that its output impedance at the frequency of the power network has the value zero. This powerline carrier control system, however, has the drawback that the powerline carrier control transmitter must be designed for the line-frequency current which is coupled via the transformers to the transmitter side. This means that the cost of the powerline carrier control transmitter, e.g. the thyristor inverter, in this case is considerable.
It is an object of the present invention to provide a powerline carrier control system of the kind mentioned at the outset in such a way that its coupling unit (series or parallel coupling unit) is considerably simplified and the cost of such a coupling unit, thereby, is lowered considerably without appreciably increasing the cost of the power section of the powerline carrier control transmitter. This should be possible, particularly in a powerline carrier control system with a parallel coupling unit.